The present invention is generally directed to particle beam testing methods. More specifically, the invention is directed to a particle beam testing method wherein a detector is used to detect secondary electrons emitted from a specimen and to generate a detector current that in turn is used to generate a retarding voltage or countervoltage that is then fed back to a field placed in the path of the secondary electrons as a result of which an electrical characteristic of the specimen can be obtained.
A particle beam testing method of this type is described in an article by Furakawa et al., published in the proceedings of SPIE--The International Society of Optical Engineering, Vol. 632, Electron-Beam, X-Ray & Ion-Beam Techniques for Sub-Micrometer Lithographies V, Mar. 11-12, 1986, pp. 232-236 and entitled "Quantitative voltage measurement by a software closed loop technique in electron beam testing." In this article, there is described an electron beam testing method for quantitative measurement of a specimen utilizing a software closed loop technique. For example, a control loop is used to feed-back a retarding voltage or countervoltage to a retarding electrode such that a detector current becomes identical to a prescribed reference current and such that there is a linear relationship between a specimen voltage to be measured and the set retarding voltage or countervoltage. The teachings of that article are incorporated herein by reference.
In pertinent part, the article describes that the software closed loop technique is used for quantitative voltage measurement and electron beam testing for LSI's. In that regard, the retarding voltage of an energy analyzer is controlled iteratively by a computer to reduce difference between a slice level and a secondary electron signal to 0. The voltage is determined by the retarding voltage at the cross-point of the slice level and the energy distribution curve.
As acknowledged in that article, it takes a long time, typically several minutes, to obtain a voltage wave form with an energy analyzer, if an energy distribution curve is required for every sampling phase. Typically, in hardware closed loop techniques, complex feedback circuits are required and these lack flexibility for correcting the influence of beam intensity drift and specimen contamination.
In another article by Fujioka et al., entitled "An open-loop spectroscopy for quantitative waveform measurements with the scanning electron microscope" published in Vol, 18 in the Journal of Physics and Education Science Instrumentation, pp. 284-285 (1984), there is described another method for quantitative measurements employing another particle beam testing method. The teachings of that article are incorporated herein by reference.
As described in that article, a retarding field energy analyzer system is provided for quantitative wave form measurement which uses no feedback loop. The system is controlled with a minicomputer to allow sampling and storing of retarding curves (S-curves) at each stroboscopic sampling phase.
By reading the retarding voltages at an arbitrarily specified traverse position or slice level of the spectrometer or S-curves, according to the authors, one can get a quantitative relation between the specimen voltage and the sampling phase.